Columnar, thallium-doped CsI scintillator screens have become the benchmark in digital radiography. Thallium-doped CsI screens provide an excellent combination of high scintillation efficiency and high x-ray absorptivity over wide energy ranges, while the columnar structure permits the screens to maintain a high spatial resolution at high x-ray stopping power. As a result, from an image quality perspective, columnar thallium-doped CsI screens have set the standard inmost radiographic applications.
While columnar thallium-doped CsI screens have the potential to provide the highest spatial resolution needed for radiographic applications, achieving this potential has been difficult given the practical demands of radiography and the mechanical and environmental fragility of CsI-based materials. For example, CsI is highly water soluble and hygroscopic. Any scintillator panels made with CsI:Tl must be maintained in a sealed, low humidity environment to avoid attracting water that can negatively affect luminescence. CsI:Tl structures are also mechanically fragile, requiring special handling procedures during and after manufacture such as complete enclosure in shock resistant containers. As a result, production (and end product) costs are quite high in applications that have successfully realized the image quality benefit of thallium-doped CsI scintillator screens.
As a result, numerous attempts have been made over the past several decades to develop scintillator screens having a columnar structure using materials that would offer better mechanical and environmental stability. In order to create mechanically robust alternatives to columnar CsI scintillator screens, it is helpful to discuss the methods that could be used to fabricate a scintillator screen that satisfies the basic physics of total internal reflection. It is the concept of total internal reflection that enables columnar CsI scintillator screens to minimize the divergence of the optical radiation generated upon x-ray irradiation, and thus maximizes the spatial resolution of the screens. The various approaches that have been used to explore alternatives to columnar CsI screens thus far have been predominantly additive in nature (e.g., creating fibers containing scintillator materials and subsequently assembling the fibers into scintillator screens, creating microwells or microvoids, which are subsequently filled with a scintillator material)—of which the dominant approach has been the microwell technology. The fiber approach has not been particularly successful thus far, due to practical challenges in particle loading and fiber extrusion of particle-loaded material, while the microwell technology has faced a more fundamental challenge in establishing the conditions required for total internal reflection. Also, in the case of fibers, it has been found difficult to assemble the fibers into a bundle with a form factor that is useful for practical applications. In the case of the microwells, the process of filling of the microwells with the scintillator particles introduces air pockets, which results in regions of the microwells having a lower refractive index (than that of the walls of the microwells), and the conditions for total internal reflection are compromised in these regions. The concentration of these air pockets is non-uniform from microwell to microwell, which result in the deterioration in the optical performance of the scintillating screen. As a result, none of these approaches have successfully created a practically useful scintillator screen that approaches the image quality of columnar thallium-doped CsI scintillator screens.
While prior techniques may have achieved certain degrees of success in their particular applications, there is a need to provide, in a cost-friendly manner, patterned scintillator panels having not only image quality approaching that of CsI-based scintillator panels but also excellent mechanical and environmental robustness. The subtractive approach of patterning a continuous (non-patterned) scintillator screen as described in this disclosure overcomes the limitations of the additive methods used in previous approaches to creating a patterned scintillator screen.